US measles surge this year is biggest since 1996

May 24, 2011 (CIDRAP News) – The United States has had 118 measles cases so far this year, the most for this date since 1996, and close to 90% of them are linked to cases in other countries, the Centers for Disease Control and Prevention (CDC) reported today.

"The unusually large number of importations into the United States in the first 19 weeks of 2011 is related to recent increases in measles in countries visited by US travelers," the CDC said in an early-release article from Morbidity and Mortality Weekly Report (MMWR).

The 118 cases were reported by 23 states and New York City between Jan 1 and May 20, the CDC said. By comparison, a median of only 56 cases was reported for all of each year from 2001 through 2008.

Of the 118 cases, 105 (89%) were associated with cases abroad. Forty-six cases were imported by US travelers and foreign visitors, and another 49 were epidemiologically linked to imported cases. The other 10 had no known epidemiologic links to imported cases, but the viral genotype suggested recent importation, the report says.

Cases were most commonly imported from countries in the World Health Organization's European and Southeast Asian regions, the CDC said. It noted that 33 countries in the European region have had recent increases in measles, with France reporting about 10,000 cases in the first 4 months of this year.

Forty-seven of the 118 US case-patients (40%) were hospitalized, the report says. Though nine patients had pneumonia, none had encephalitis and none died.

The overwhelming majority—105 (89%)—of the 118 patients had not been vaccinated, the CDC said. Of 45 US residents between 1 and 19 years old who had the illness, 39 were unvaccinated, including 24 whose parents claimed a religious or personal exemption.

Nine outbreaks (defined as three or more related cases) accounted for 58 of the 118 cases, with six outbreaks traced to imported cases. The biggest one consisted of 21 cases in a Minnesota population (identified in local media reports as Somalis) in which many children were unvaccinated because of parental concerns about vaccine safety, the report says.

The CDC notes that measles can be a severe disease, as was shown between 1989 and 1991, when the nation had more than 100 deaths out of 55,000 cases.

The virus is also so infectious that up to 90% of susceptible people will get sick after exposure. "Maintenance of high 2-dose MMR [measles, mumps and rubella] vaccination coverage is the most critical factor for sustaining elimination," the report says. "For measles, even a small decrease in coverage can increase the risk for large outbreaks and endemic transmission, as occurred in the United Kingdom in the past decade."

All travelers at least 6 months old are eligible for MMR vaccination and should be vaccinated before traveling abroad, the CDC advises. The agency also says healthcare providers should suspect measles in patients "with a febrile rash illness and clinically compatible symptoms (eg, cough, coryza, and/or conjunctivitis) who have recently traveled abroad or have had contact with travelers."